This invention relates to the screening of wood chips and, especially, to the removal of the pin chips and to the dosing of them back into the process.
The objective of the chip screening is to free the chips from such shares of small-size and large-size fractions that could have a detrimental effect on the pulp making process. In the screening process, the large-size fractions (too big and too thick chips) are usually treated by means of a rechipper, for example, into smaller particles or crushed by means of a roller so press into a form more suitable for the process. The finest fraction (the sawdust) causes problems in most processes, which is why every effort is made to purify the chips from it as well as possible. The next biggest fraction, the so-called pin chips, as such, constitutes quite a good raw material in terms of fibres, but a high content of pin chips is disadvantageous to some processes, e.g. to continuously operating digesters, as it may cause malfunctions in the process (e.g. blocks).
For the removal of pin chips, the same kind of devices are used as for the removal of sawdust, i.e. flat screens, vibrating screens, roll screens, disc screens and so forth. To the screening elements themselves, however, changes are made because of the larger particle size. Screening plants known in the prior art are described, for example, in Finnish patent specifications 79251 and 90019. If the subsequent process (a chemical or a mechanical pulp making process) sets strict restrictions on the maximal amount of pin chips, and if it is possible that the pin content of the material to be fed is high, the pin chips are usually separated by screening and stored in a separate bin. Then, the pin chips are dosed into the accepted fraction fed to the subsequent process, using a constant ratio. The plants designed for this purpose are reliable as such but represent, from an economical point of view, a considerable extra investment. On an average, the amount of pin chips among the chips to be screened is usually larger than the allowable amount, especially when the chips are bought from sawmills. The method according to the invention provides the same result at substantially lower costs. The pin chips are separated in the screening process but a maximum desired amount is dosed back among the chips due for the process immediately after the separation. Thus, no intermediate storage of the pin chips, and, consequently, no pin bin with auxiliary devices are needed in the method according to the invention. The screening can be optimised.